Introduction

Think of pokemon yellow as creating a little universe with certain
rules. Inside that universe, you can buy items, defeat rival trainers,
and raise your pokemon. But within that universe, you are bound by the
rules of pokemon. You can't build new buildings, or change the music,
or change your clothes.. There are some games (like chess), where it
is not possible to alter the rules of the game from within the
game. No matter what moves you make in chess, you can never change the
rules of the game so that it becomes checkers or basketball. The point
of this run is to show that you CAN change the rules in pokemon
yellow. There is a certain sequence of valid actions (like walking
from one place to another or buying items) that will allow you to
transform pokemon yellow into Pacman, or Tetris, or Pong, or a MIDI
player, or anything else you can imagine.

Background

The speedrun (http://tasvideos.org/2913S.html) by Felipe Lopes de
Freitas (p4wn3r), beats pokemon yellow in only 1 minute and 36
seconds. It does it by corrupting the in-game item list so that he can
advance the list past its normal limit of 20 items. The memory
immediately after the item list includes the warp points for the
current map, and by treating that data as items and switching and
dropping them, he can make the door from his house take him directly
to the end of the game.

When I first saw that speedrun, I was amazed at how fast pokemon
yellow could be beaten, and that it was possible to manipulate the
game from the inside, using only the item list. I wondered how far I
could extend the techniques found in p4wn3r's run.

The gameboy is an 8 bit computer. That means that ultimately, anything
that happens in pokemon is a result of the gameboy's CPU reading a
stream of 8 bit numbers and doing whatever those numbers mean. For
example, in the gameboy, the numbers:

62 16 37 224 47 240 37 230 15 55

mean to check which buttons are currently pressed and copy that result
into the "A" register. With enough numbers, you can spell out an
interactive program that reads input from the buttons and allows you
to write any program you want to the gameboy. Once you have assembled
such a program and forced the game to run it, you have won, since you
can use that program to write any other program (like Tetris or
Pacman) over pokemon yellow's code. I call a program that allows you
to write any other program a "bootstrapping program". So, the goal is
to somehow get a bootstrapping program into pokemon yellow and then
force yellow to run that program instead of its own.

How can we spell out such a program? Everything in the game is
ultimately numbers, including all items, pokemon, levels, etc. In
particular, the item list looks like:

So, if we can get the right items in the right quantities, we can
spell out a bootstrapping program. Likewise, when writing the
bootstrapping program, we must be careful to only use numbers that are
also valid items and quantities. This is hard because there aren't
many different items to work with, and many machine instructions
actually take 2 or even 3 numbers in a row, which severely restricts
the types of items you can use. I ended up needing about 92 numbers to
implement a bootstrap program. Half of those numbers were elaborate
ways of doing nothing and were just there so that the entire program
was also a valid item list.

The final part of the hack is getting pokemon yellow to execute the
new program after it has been assembled with items. Fortunately,
pokemon keeps a number called a function pointer within easy reach of
the corrupted item list. This function pointer is the starting point
(address) of a program which the game runs every so often to check for
poison and do general maintenance. By shifting an item over this
function pointer, I can rewrite that address to point to the
bootstrapping program, and make the game execute it. Without this
function pointer, it would not be possible to take over the game.

The Run

Pallet

I start off and name my rival Lp/k. These characters will eventually be
treated as items and shifted over the function pointer, causing it to
execute the bootstrapping program that will soon be constructed. I
start the run the same as p4wn3r's and restart the game while saving,
so that the pokemon list is corrupted. By switching the 8th and 10th
pokemon, I corrupt the item list and can now scroll down past the 20th
item. I shift items around to increase the text speed to maximum and
rewrite the warp point of my house to Celadon Dept. Store. (p4wn3r
used this to go directly to the hall of fame and win the game in his
run.) I deposit many 0x00 glitch items into the PC from my corrupted
inventory for later use. Then, I withdraw the potion from the
PC. This repairs my item list by overflowing the item counter from
0xFF back to 0x00, though the potion is obliterated in the process. I
then take 255 glitch items with ID 0x00 from the computer into my
personal items.

Celadon Dept. Store

Leaving my house takes me directly to Celadon Dept. store, where I
sell two 0x00 items for 414925 each, giving myself essentially max
money. I hit every floor of the department store, gathering the
following items:

After gathering these items, I deposit them in the appropriate order
into the item PC to spell out my bootstrapping program. Writing a full
bootstrap program in one go using only items turned out to be too
hard, so I split the process up into three parts. The program that I
actually construct using items is very limited. It reads only from the
A, B, start, and select buttons, and writes 4 bits each frame starting
at a fixed point in memory. After it writes 200 or so bytes, it jumps
directly to what it just wrote. In my run, I use this program to write
another bootstrapping program that can write any number of bytes to
any location in memory, and then jump to any location in memory. This
new program can also write 8 bits per frame by using all the
buttons. Using this new bootstrap program, I write a final
bootstrapping program that does everything the previous bootstrapping
program does except it also displays the bytes it is writing to memory
on the screen.

Finale

After completing this bootstrapping program, I go to the Celadon
mansion, because I find the metaness of that building to be
sufficiently high to serve as an exit point for the pokemon
universe. I corrupt my item list again by switching corrupted pokemon,
scroll down to my rival's name and discard until it is equal to the
address of my bootstrapping program, and then swap it with the
function pointer. Once the menu is closed, the bootstrapping program
takes over, and I write the payload....

Other comments

The entire video was played by the computer using bots. I used
functional programming to write search programs over different
possible game states to find the most efficient way of performing
general actions. Some interesting things I developed but didn't use
were pretty printing functions to display the game's internal data
structures, and an "improbability drive" that forces improbable events
to happen automatically using search.

This script walks from the Viridian City pokemon store to Oak's
Lab in the most efficient way possible. The walk-thru-grass function
guarantees that no wild battles will happen by manipulating the game's
random number generator.

This script calculates the fastest sequence of key presses to deposit
the requested items into a PC, assuming that the character starts out
in front of a computer.

Other Comments

The final payload program is multiple programs. I created a reduced
form of MIDI and implemented it in gameboy machine language. Then I
translated a midi file from http://www.everyponysings.com/ into this
reduced MIDI language. The payload program contains both the music
data and the MIDI interpreter to play that data. The picture works in
a similar way. There is code to translate a png file into a form that
can be displayed on a gameboy, and other code to actually display that
image. Both the image and the display code are also written by the
final bootstrapping program. Even though my final payload is rather
simple, you can write any program at all as the payload. The source
for the sound and image displaying code is at
http://hg.bortreb.com/vba-clojure

This entire project is open source and I encourage anyone who wants to
take the code and play around!